7. The compasses, next to the T square and triangles, are used more than any other instrument. A pencil and pen point are provided, as shown in Fig, 8, either of which may be inserted into a socket in one leg of the instrument, for the drawing of circles in pencil or ink. The other leg is fitted with a needle point, which acts as the center about which the circle is drawn.
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In all good instruments, the needle point itself is a separate piece of round steel wire, held in place in a socket provided at the end of the leg. The wire should have a square shoulder at its lower end, below which a fine, needle-like point projects. The Lengthening bar, also shown in the figure, is used to extend the leg carrying the pen and the pencil points when circles of large radii are to be drawn.
The joint at the top of the compasses should hold the legs firmly in any position, and at the same time should permit their being opened or closed with one hand. The joint may be tightened or loosened by means of a screwdriver or wrench, which accompanies the compasses.
It will be noticed in Fig. 8 that each leg of the compasses is jointed; this is done so that the compass points may always be kept perpendicular to the paper when drawing circles, as in Fig. 11.
The style of compasses shown in Fig. 8 have what is called a tongue joint, in which the head of one leg has a tongue, generally of steel, which moves between two lugs on the other leg. Another common style of joint is the
